Nanomechanical characterization of IMCs formed in SAC solder joints subjected to isothermal aging

Isothermal aging of lead-free Sn-Ag-Cu (SAC) solder joints leads to growth of intermetallic (IMC) particles in the solder bulk as well as growth of intermetallic layers at the joint interfaces with copper bond pads. Fracturing near the interfacial IMC layers is often found to be the primary reason for failures caused by drop impacts. The IMCs in SAC joints are primarily Ag₃Sn and Cu₆Sn₅ binary compounds. Cu-NiSn based ternary IMCs can also form at the interface of Ni containing surface finish (i.e. ENIG) and SAC solder. The mechanical properties of these IMCs are very different than those of the Sn rich dendrites and Cu pads. Nanoindentation (NI) techniques are powerful tools to characterize mechanical properties of small particles and thin layers. In this study, the mechanical behaviors of IMC particles and layers in SAC solder joints have been characterized using nanoindentation. SAC BGA solder joints were first aged for 6 months at T = 125 °C. Test samples were subsequently prepared by cross-sectioning the aged solder joints, and then molding them in epoxy and polishing them to prepare the joint surfaces for microscopy and nanoindentation. Intermetallics formed in the bulk solder region, copper pad and SAC solder interface, and ENIG plating finish and SAC solder interface were observed and detected using SEM and the energydispersive x-ray spectroscopy (EDX) technique. The same intermetallics were then indented to measure their room temperature mechanical properties including the elastic modulus, hardness, and creep strain rate. To ensure the indentation occurred at the desired phase, SPM imaging was done prior and after the indentations. As expected, the measured properties of the IMCs were significantly higher than the Sn-matrix forming the solder joints. The measured elastic modulus and hardness values were 98.1 ± 6.3 and 5.80 ± 0.70 GPa for the Cu₆Sn₅ layers at the joint and copper bond pad interfaces, 132.5 ± 4.5 and 8.58 ± 1.13 GPa for the Cu_1-x Ni_x)₆Sn₅ layers at the joint and ENIG plating finish interfaces, and 75.2 ± 4.0 and 3.085 ± 0.50 GPa for the Ag₃Sn IMC particles in the solder joint bulk. These are all much higher than the values of 42.7 ± 2.3 and 0.21 ± 0.12 GPa measured for the β-Sn phase in the solder joint bulk. Creep testing performed at 25 °C revealed that the (Cu_1-xNi_x)₆Sn₅ IMC layers had the lowest steady state secondary creep rate of 1.088 × 10^-3 sec^-1, whereas the Cu₆Sn₅ layers had a creep rate of 1.66 × 10^-3 sec^-1, and the Ag₃Sn IMC particles had a creep rate of 2.34 × 10^-3 s^-1. The creep stress exponents were evaluated from log-log plots of the strain rate vs. applied stress data.